The hat was crafted by one of Princess Diana's favourite milliners, Gina Foster.
Osborne also credited the coat's design to a New Zealander when it was made by British designer Catherine Walker.
"She is wearing a New Zealand designer, which I really appreciate as well because she really thinks about where she is going and the price points of the outfit so she is never too inaccessible and that's what I love about her style in general," Osborne said.
Guest host Alyssa Milano said the Duchess showed style and poise and stylist George Kotsiopoulos described her look as "timeless".
"I love what she is wearing it is timeless, this is elegant, it's not old-fashioned, it's not modern."
Despite stiff competition from Hollywood royalty Taylor Swift, host Joan Rivers crowned Kate as overall best-dressed.
Kate's fashion choices have been one of the biggest talking points of the 10-day visit to New Zealand, and have been described by one commentator as further evidence of the Duchess as a fashion icon.
Sunday Telegraph fashion writer Belinda White said Kate's fashion choices were a nod to Diana, Jackie Kennedy and Audrey Hepburn.
More glitz and glam are expected when she goes to Australia this week.
"Come Wednesday, when the tour moves on to Australia, expect to see her in her style element," she wrote.
"If there's one thing no one can fault this girl on, it's her ability to outshine any Hollywood pro in the field of red-carpet dressing.
"Expect knockout, embellished glamour and floor-sweeping Grecian gowns worthy of Elizabeth Taylor and Grace Kelly."
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